This invention relates to an externally pressurized gas bearing spindle mounted in a rotary atomizing head type electrostatic painting machine to support a main spindle carrying a paint spray head in a non-contact manner by means of an externally pressurized gas bearing.
Heretofore, for painting of automotive parts, electric parts, etc., various kinds of electrostatic painting machines have been used in which paint is atomized and electrostatically charged with a negative charge to cause it to adhere to surfaces to be painted using electrical attraction.
With such an electrostatic painting machine, in order to obtain a uniform painted surface by reducing the particle diameter of the atomized paint as much as possible, it is required to rotate the atomizing head for paint at high speed. In order to cope with this requirement, a structure is usually employed in which a rotary shaft of a spindle on which is mounted the atomizing head is supported by an externally pressurized gas bearing in a non-contact manner.
FIG. 9 shows a first example of a conventional spindle for a rotary atomizing head type electrostatic painting machine. In this spindle, a high voltage generator 2 for generating a negative high voltage is electrically connected to a housing 1. Inside of the housing, an air passage 3 with an air inlet 3a connected to a compressed air supply source (not shown) is formed. In an internal hole 1a in the housing 1, a plurality of air supply nozzles 4, 5, 5xe2x80x2 communicating with the air passage 3 are formed.
In the internal hole 1a in the housing 1, a main spindle 7 and a rotor 8 of a turning force generator (e.g. air turbine) provided at the rear end of the main spindle for rotating the main spindle 7 are inserted so as to form bearing gaps 6, 6xe2x80x2. Between the periphery of the main spindle 7 and the air supply nozzles 4, a journal air bearing portion 9 is formed. Between the rotor 8 and the air supply nozzles 5, 5xe2x80x2, thrust air bearing portions 10, 10xe2x80x2 are formed. When compressed air is supplied from the air supply nozzles 4, 5 and 5xe2x80x2 into the bearing gap 6, 6xe2x80x2, due to the pressure of the air, the main spindle 7 and the rotor 8 are supported in a floating state and out of contact with the housing 1, so that the externally pressurized gas bearing comprising the journal air bearing portion 9 and the thrust air bearing portion 10, 10xe2x80x2 reveals its function.
At the front end of the main spindle 7, a paint atomizing head 13 for atomizing and spraying paint to the surroundings by the rotation of the main spindle 7 is fixed by suitable means such as screws. A conduit 11 having an injection nozzle 17 at its tip to feed paint to the spray head 13 is inserted into the main spindle 7 out of contact with the main spindle.
With this spray head 13, paint injected from the injection nozzle 17 into a paint introducing space 15 defined by a partitioning wall 16 passes through a flow-out hole 18 and is introduced to an inner peripheral surface 14 of the spray head 13 by high-speed rotation of the spray head 13, and scattered in a mist to the surroundings by centrifugal force. If the spray head 13 is in a negatively charged state, the paint flowing along the inner peripheral surface 14 is electrostatically charged with a negative charge.
Around the rotor 8 at the rear end of the main spindle 7, a plurality of turbine blades 19 are arranged. In the housing 1, compressed air blow-out nozzles 20 for blowing compressed air against the turbine blades 19 is formed. Also, a compressor 21 is coupled to the blow-out nozzle 20. With this structure, when compressed air is blown against the turbine blades 19 through the blow-out nozzle 20, turning force is given to the turbine blades 19. Thus, the main spindle 7, which is supported in a floating state, rotates at a high speed.
FIG. 10 shows a second example of a prior art spindle for an electrostatic painting machine using an air turbine as a driving means. It has a main spindle 7 and a rotor 8 provided at the rear end of the main spindle. The rotor 8 is formed with a plurality of recesses 19xe2x80x2 in its outer periphery and a housing 1 is formed with an air blow-out nozzle 20 at a position opposite to the rotor 8. Compressed air is blown from the nozzle 20 to rotate the main spindle 7. A paint spray head (not shown) is mounted to the front end of the main spindle 7.
The main spindle 7 and the rotor 8 integral with the main spindle are rotatably supported in a non-contact manner by bearing sleeves 51, 52 and first and second housings 53, 54 by a journal bearing portion 9 and thrust bearing portions 10, 10xe2x80x2 formed by the bearing sleeves 51, 52 and the first and second housings 53, 54.
The journal bearing portion 9 and the thrust bearing portion 10 are formed by mounting the bearing sleeve 51 in the first housing 53 by shrinkage fitting, press fitting or bonding. Similarly the thrust bearing portion 10xe2x80x2 is formed by mounting the bearing sleeve 52 in the second housing 54 by shrinkage fitting, press fitting or bonding.
The bearing sleeves 51 and 52 are made of a copper alloy which is an ordinary air bearing material. They are formed with air supply nozzles 4, 4xe2x80x2 for the journal bearing and air supply nozzles 5, 5xe2x80x2 for the thrust bearing.
Between the first and second housings 53, 54 is mounted a third housing 55. The first to third housings 53-55 are secured together by bolts or the like. The first to third housings (forming the entire housing 1) are arranged in a painting machine casing 56 made of a resin through elastic members 57. The housing 1 is normally made of a light metal such as aluminum alloy for lighter weight.
The first and second housings 53, 54 are formed with annular grooves 58-60 which communicate with air supply nozzles 4, 4xe2x80x2 and 5, 5xe2x80x2 formed in the bearing sleeves 51, 52. Also, air passages 3, 3xe2x80x2, 62, 61 communicating with these annular grooves 58-60 are formed in the housings 53, 54 and 55.
The air passage 63 communicating with an air supply port 3a is formed in the painting machine casing 56. Also, the third housing 55 is formed with the compressed air blow-out nozzles 20 opening opposite to the recesses 19xe2x80x2 of the rotor 8 in a tangential direction.
Compressed air supplied from the air passage 63 in the painting machine casing 56 is supplied from the air supply port 3a of the first housing 53 through the annular groove 59 and the air passage 3 to the annular grooves 58, 59 and blown out to the bearing surface through the air supply nozzles 4, 4xe2x80x2 of the bearing sleeve 51 so that the journal bearing portion 9 supports the main spindle 7 in a radial direction.
Also, compressed air is blown out through the annular groove 59 of the first housing 53 from the air supply nozzle 5 to the bearing surface of the thrust bearing portion 10xe2x80x2. The compressed air is also supplied through the air passages 3xe2x80x2, 62 and 61 of the first, third and second housings 53, 55 and 54, respectively, to the annular groove 60 and blown from the air supply nozzle 5xe2x80x2 to the bearing surface of the thrust bearing portion 10xe2x80x2. Thus the thrust bearing portions 10, 10xe2x80x2 support the main spindle 7 in an axial direction.
On the other hand, compressed air supplied from an air passage 64 of the painting machine casing 56 is blown from the air blow-out nozzles 20 of the third housing 55 toward the recesses 19xe2x80x2 of the rotor 8 of the main spindle 7 in a tangential direction. After applying a turning force to the main spindle 7, compressed air is discharged through an exhaust port (not shown) out of the housing 1. At one end (right-hand end in FIG. 10) of the main spindle 7, a paint spray head (not shown) is mounted to atomize paint for electrostatic painting.
With the structure of the first prior art spindle, though depending upon applied voltage, its polarity, current, bearing gap, material used, etc., a discharge phenomenon sometimes occurs in the bearing gap 6, so that sputtering occurs. If sputtering occurs, atoms forming the housing 1 or the main spindle 7 with the bearing gap 6 therebetween will jump out, thus changing the bearing gap 6, so that the support state of the main spindle 7 becomes unstable. This may cause the main spindle 7 to come into contact with the housing 1.
An object of this invention is to provide a spindle for an electrostatic painting machine having improved bearing endurance by preventing occurrence of sputtering at bearing gaps without being affected by such conditions as the applied voltage, polarity, current, bearing gaps, etc.
In the second example of the prior art spindle shown in FIG. 10, the housing 1 is ordinarily formed of a light metal such as aluminum alloy for lighter weight and the bearing sleeves 51 and 52 are ordinarily formed of a copper alloy. In such a case, since copper alloy has a larger density than aluminum alloy, the volume ratio of the housing 1 to the bearing sleeves 51 and 52 is ordinarily set so as to reduce the weight of the entire housing assembly.
On the other hand, in recent years, in electrostatic painting, change from an organic solvent paint to a water-soluble paint is advancing in view of environmental problems. But, if a water-soluble paint is used, the housing made of an aluminum alloy could corrode. One solution to this problem is to make the housing of stainless steel. But this change without changing the volume ratio of the housing to the bearing sleeves would increase the weight of the entire housing assembly substantially.
Also, the requirement for higher speed of the main spindle is increasing to provide for higher painting performance. Because a water-soluble paint has a higher viscosity than an organic-solvent paint, the requirement for higher speed of the spindle is accordingly high. With higher speed of the spindle, increased endurance of the bearing portion is required. In this connection, as a material which is less susceptible to damage due to contact with the bearing portion during high speed rotation, there is graphite, which has excellent self-lubricity and has in recent years been used as an air bearing material.
However, if the bearing sleeves 51, 52 are formed of graphite, anode corrosion could occur at the bonding surface with the housing made of an aluminum alloy. This problem is particularly serious if a water-soluble paint is used. Thus it is difficult to use the bearing sleeves made of graphite and the housing made of an aluminum alloy in contact with each other.
It is conceivable to form the housing and the bearing sleeves integral and make the united member of graphite. But graphite has a small elastic coefficient, is susceptible to deformation upon chucking during machining and, when used for the bearing sleeves, makes it difficult to machine the bearing surface with high accuracy. Also, with an electrostatic painting machine, the spindle has to be mounted on and removed from the painting machine during maintenance work. If the housing is made of graphite, it is liable to get marred or chipped during mounting and removal.
Another object of this invention is to provide a spindle which has improved corrosion resistance and bearing endurance during high speed rotation.
According to this invention, there is provided a spindle for an electrostatic painting machine comprising a stationary member, a main spindle mounted in the stationary member, a journal bearing portion for statically supporting the main spindle in a radial direction relative to the stationary member, a thrust bearing portion for statically supporting the main spindle in a thrust direction relative to the stationary member, a paint spray head mounted to one end of the main spindle for atomizing paint, and a current-carrying member having electrical conductivity retained relative to the stationary member and mounted opposite to the main spindle through a gap set to be smaller than gaps of the journal and thrust bearing portions.
With this arrangement, since discharge occurs concentratedly at the portion of the current-carrying member having a gap smaller than the gaps of the bearing portions, occurrence of sputtering at the bearing gaps of the externally pressurized gas bearing portions is prevented. Also, since the current-carrying member is not in contact with the main spindle, there will be no influence on the rotary performance of the main spindle, which is supported by the externally pressurized gas bearing portions.
The current-carrying member may be mounted on the stationary member so as to be movable toward and away from the main spindle, the current-carrying member being brought into contact with the main spindle with a predetermined biasing force, and the gap being formed by floating the current-carrying member off the main spindle by dynamic pressure produced by the rotation of the main spindle. With this arrangement, by setting the dynamic-pressure generating conditions, it is possible to easily form a small gap between the current-carrying member and the main spindle. Also, it is possible to maintain the gap constant. Further, by forming the gap small, it is possible to make the discharge energy small. Thus it is possible to suppress influence of sputtering at the gap portion.
Also, an adjusting means may be provided to adjust the biasing force from outside of the stationary member. A wedge-like space for producing dynamic pressure may be formed at the contact surface between the current-carrying member and the main spindle.
Further, the current-carrying member may be formed so as to be annular or arcuate, and it may be provided on the journal bearing portion. The current-carrying member may be formed rod-like and it may be provided on the thrust bearing portion. Further, the wedge-like space may be formed by initial wear during running-in of the main spindle. The current-carrying member may be formed of graphite.
According to the present invention, there is also provided a spindle for an electrostatic painting machine comprising a main spindle, a bearing sleeve mounted on the main spindle, a housing mounted on the bearing sleeve, and a paint spray head fixedly mounted to one end of the main spindle for atomizing paint, the bearing sleeve and the housing forming a bearing portion for rotatably supporting the main spindle by static pressure, the bearing sleeve being made of a brittle material having self-lubricity, the housing being made of a metallic material having corrosion resistance.
The bearing portion comprises a journal bearing portion for statically supporting the main spindle in a radial direction relative to the housing and a thrust bearing portion for statically supporting the main spindle in a thrust direction relative to the housing.
With this arrangement, by making the housing of a metallic material having corrosion resistance such as stainless steel and making the bearing sleeves of a brittle material having self-lubricity such as graphite, it is possible to provide a spindle suitable for an electrostatic painting machine which uses a water-soluble paint.
By setting the specific weight of the bearing sleeves to one third or less that of the housing and making the volume of the bearing sleeves larger than that of the housing, it is possible to reduce the weight of the entire housing assembly comprising the housing and the bearing sleeves.
It is preferable to form in the bearing sleeves air passages for compressed air supplied to the journal and thrust bearing portions because this reduces the volume of the housing.
This invention is applicable to a structure in which a turbine for driving is formed integrally on the main spindle. Also, it is preferable that the turbine defines the thrust bearing portion and is formed on the outer periphery of the rotor integrally formed on the main spindle. Also, this invention is suitable for an electrostatic painting machine which uses a water-soluble paint.